1. Field of the Invention
The present invention relates to MEMS (Micro Electro-Mechanical System) transducer array chips, and in particular to condenser microphone array chips fabricated with arrays of MEMS condenser microphones.
The present application claims priority on Japanese Patent Application No. 2010-220575, the entire content of which is incorporated herein by reference.
2. Description of the Related Art
It is conventionally known that a single microphone may include a plurality of MEMS condenser microphones. Patent Documents 1-3 disclose microphones in which a plurality of condenser microphones is formed on a single chip. Additionally, it is conventionally known that an S/N ratio can be improved using a plurality of condenser microphones connected in a parallel manner or in a cascaded manner.
A single substrate fabricated with a microphone array including a plurality of MEMS condenser microphones suffers from a problem that warp/bend (hereinafter, referred to as “warpage”) thereof increases as the chip area increases due to the array structure, the reason of which will be described below.
Condenser microphones are fabricated in such a way that a diaphragm (serving as a vibrating electrode) and a plate (serving as a static electrode) are composed of conductive polycrystal silicon and formed on a substrate, wherein silicon oxide films composed of an insulating material are interposed in layers between the substrate, diaphragm, and plate. Silicon oxide films can be increased in thickness by way of the low-stress plasma CVD method (where CVD stands for Chemical Vapor Deposition). Annealing is needed to increase thickness to secure an adequate sensitivity and alleviate stress applied to the diaphragm. Annealing is able to alleviate tensile stress applied to the diaphragm composed of polycrystal silicon, thus demonstrating high-sensitive acoustic performance. However, it is revealed that stress applied to silicon oxide films be changed into highly compressive stress due to annealing, which may lead to large warpage of a substrate and buckling destruction of deposited films. In addition, it is difficult to achieve desired acoustic performance when silicon oxide films are not thickened to prevent warpage and buckling destruction.